A prior art example of this kind of key top is shown in FIGS. 3 and 4.
In these figures a rectangular tube-shaped engaging protrusion 3, which is inserted in a hollow stem 10 movable up and down with respect to the main body of a switch not shown in the figures, is disposed at the approximately central portion on the bottom surface of a top plate 2 of a key top 1, protruding therefrom. Around this engaging protrusion 3 there are disposed four ribs 4. Each of the ribs 4 projects perpendicularly from the outer wall surface of the engaging protrusion 3. The ribs 4 act to reinforce the top plate 2 from the force encountered when the key top is pushed down.
In the prior art key top 1 described above, in the case where an edge of the top plate 2 is pushed down, a force acts laterally on the engaging protrusion 3 through the hollow stem 10. For example, when one end is pushed as indicated by an arrow in FIG. 4, stress is concentrated at the point indicated by A in the engaging protrusion 3.
For this reason, the risk is high that a crack may form at the portion indicated by A, ultimately severing the engaging protrusion 3 from the rest of the key top.